Microminiature field emitters are well known in the microelectronics art. These microminiature field emitters are finding widespread use as electron sources in microelectronic devices. For example, field emitters may be used as electron guns in flat panel displays for use in aviation, automobiles, workstations, laptops, head wearable displays, heads up displays, outdoor signage, or practically any application for a screen which conveys information through light emission. Field emitters may also be used in non-display applications such as power supplies, printers, and X-ray sensors.
When used in a display, the electrons emitted by a field emitter are directed to an cathodoluminescent material. These display devices are commonly called Field Emitter Displays (FEDs). A field emitter used in a display may include a microelectronic emission surface, also referred to as a "tip" or "microtip", to enhance electron emissions. Conical, pyramidal, curved and linear pointed tips are often used. Alternatively, a flat tip of low work function material may be provided. An emitting electrode typically electrically contacts the tip. An extraction electrode or "gate" may be provided adjacent, but not touching, the field emission tip, to form an electron emission gap therebetween. Upon application of an appropriate voltage between the emitting electrode and the gate, quantum mechanical tunneling, or other known phenomena, cause the tip to emit electrons. In microelectronic applications, an array of field emission tips may be formed on the horizontal face of a substrate such as a silicon semiconductor substrate, glass plate, or ceramic plate. Emitting electrodes, gates and other electrodes may also be provided on or in the substrate as necessary. Support circuitry may also be fabricated on or in the substrate.
The FEDs may be constructed using various techniques and materials, which are only now being perfected. Preferred FED's may be constructed of semiconductor materials, such as silicon. There are two predominant processes for making field emitters; "well first" processes, and "tip first" processes. In well first processes, such as a Spindt process, wells are first formed in a material, and tips are later formed in the wells. In tip first processes, the tips are formed first, and the wells are formed around the tips. There are multitudes of variations of both the well first and the tip first processes. The present invention relates to a tip first process.
The electrical theory underlying the operation of an FED is similar to that for a conventional CRT. Electrons supplied by a cathode are emitted from the tips in the direction of the display surface. The emitted electrons strike phosphors on the inside of the display which excites the phosphors and causes them to momentarily luminesce. An image is produced by the collection of luminescing phosphors on the inside of the display screen. This process is a very efficient way of generating a lighted image.
In a CRT, a single electron gun is provided to generate all of the electrons which impinge on the display screen. A complicated aiming device, usually comprising high power consuming electromagnets, is required in a CRT to direct the electron stream towards the desired screen pixels. The combination of the electron gun and aiming device behind the screen necessarily make a CRT display prohibitively bulky due to the spatial volume required for scanning.
FEDs, on the other hand, may be relatively thin. Each pixel of an FED has its own electron source, typically an array or grouping of emitting microtips. The voltage difference between the cathode and the gate causes electrons to be emitted from the microtips which are in electrical proximity with the cathode. The FEDs are thin because the microtips, which are the equivalent of an electron gun in a CRT, are extremely small. Further, an FED does not require an aiming device, because each pixel has its own electron gun (i.e. an array of emitters) positioned directly behind it.
One problem that has been encountered with FEDs is shorting and voltage leakage between the cathode and the gate along the surface of the insulator that separates the two. To help solve this problem FEDs may be provided with high aspect ratio emitter tips which tends to reduce the likelihood of this shorting and/or surface leakage. High aspect ratio emitters may have a base width to height ratio in the range of 1:1.1 to greater than 1:10. Because high aspect ratio emitters are taller for a given gate hole diameter, the insulator layer between the cathode and the gate can also be made taller, i.e. thicker.
An added benefit of high aspect ratio emitters is that they also tend to enhance the electric field concentration at the tip of the emitter, thereby making the emission of electrons easier (lower turn-on voltage).
Another problem that has been experienced with FEDs is the formation of cracked, broken, or non-uniform metal gate lines on the surface of the insulator between the gate and the cathode. The aforementioned problems with the gate lines may be attributable to an uneven surface of the insulator. The surface of the insulator may be made more even by using a spin-on insulator to fill the steps between the emitters. Spin-on insulators fill the steps or gaps between the emitters by being reflowed (liquefied) at high temperatures after being applied. After reflowing, the more even insulator may result in improved gate lines.
Previously used spin-on insulators, such as those used in Doan et al., U.S. Pat. No. 5,229,331 (Jul. 20, 1993) have been reflowed at high temperatures (preferably above 1000.degree. C.). Because of this high reflow temperature, soda lime glass, which begins to flow at 650.degree. C., cannot be used as a substrate in an FED using an insulator that is reflowed at such a high temperature.